Various devices for internal fixation of bone fragments in a human or animal body are known in the prior art. In the case of internal fixation of the spinal column or of parts of the spinal column, such devices often essentially comprise pedicle screws which are anchored by means of threads into the pedicles of the individual vertebrae to be joined, and one or more longitudinal supports which extend in the direction of the spinal column and have to be connected securely to the pedicle screws. To achieve stable anchoring of the whole implant, the pedicle screws must on the one hand be screwed securely into the pedicles and, on the other hand, be connected rigidly to the longitudinal supports.
The connection between the screw head of the pedicle screws and the longitudinal support is normally effected by means of clamp mechanisms which typically must permit a stable connection, even at different angles of the pedicle screw in relation to the longitudinal support. The clamp connection typically must be releasable so that the whole implant can be removed again without leaving large tissue openings in the area of the spinal column. Secure connections between bone screws and plates or supports are also common in other internal bone fixations. For such devices, different angles of the bone screws relative to the plate or the support are desired without adversely affecting the connections in terms of their stability.
For instance, one bone-anchoring screw and stabilizer rod connection for the internal fixation of vertebrae is known from U.S. Pat. No. 5,466,237 to Byrd. A bone-anchoring screw is provided with a screw head which is designed as a segment of a sphere on its side facing the screw shank and is convex at its end. The spherical segment part of the screw is mounted in a bore of the anchoring element, the bore comprising a concave portion which tapers toward the screw shank so that a ball-joint-type connection is obtained between the bone screw and the anchoring element. The ball-joint-type connection is blocked by tightening a nut on the anchoring element. The nut presses against the longitudinal support which has been placed in the anchoring element and which consequently presses against the terminal convex part of the screw head and thus blocks the screw head in the anchoring element. However, because of the often inexactly matching spherical surfaces on the bone screw and in the anchoring element, such a blockable ball-joint-type connection is unsuitable for taking up the forces which arise in bone fixation. In addition, such a ball-joint-type connection can only be blocked with frictional engagement.
Despite this development, a need exists for a stable connection between a bone fastener and an anchoring element which permits different angles between the screw axis and anchoring element and which may permit a form fit with suitable material pairing.